Problem: we may break a multibyte char sequence using a key 
reduced to maximum allowed length for a storage engine
(that leads to failed assertion in the innodb code, 
see also #17530). 

Fix: align truncated key length to multibyte char boundary.


mysql-test/r/innodb_mysql.result:
  Fix for bug #28125: ERROR 2013 when adding index.
    - test result.
mysql-test/t/innodb_mysql.test:
  Fix for bug #28125: ERROR 2013 when adding index.
    - test case.
sql/sql_table.cc:
  Fix for bug #28125: ERROR 2013 when adding index.
    - align truncated key length to multibyte char boundary.
    - display real key length in bytes raising warnings.